There are a number of wellbore tools which can be operated to enlarge an existing hole as the tool is rotated and advanced axially along the existing hole. Some tools with this function have a plurality of cutter assemblies distributed axially around a longitudinal axis of the tool and each cutter assembly includes a sequence of cutters which extends axially along the tool with each cutter having a leading surface facing in a direction of rotation of the tool.
One wellbore tool which may be constructed to come within this category is a reamer. A reamer may be constructed to have a fixed diameter, in which case the reamer must start cutting at the surface or at the end of an existing hole of equal or greater size. Alternatively a reamer can be constructed so as to be expandable so that it can enlarge a borehole to a greater diameter than that of the hole through which the (unexpanded) reamer was inserted.
Enlarging a borehole with a reamer may be done as a separate operation to enlarge an existing borehole drilled at an earlier time. Enlarging with a reamer may also be done at the same time as using a bottom hole assembly which has a drill bit at its bottom end. The drill bit makes an initial hole, sometimes referred to as pilot hole, and a reamer positioned at some distance above the drill bit increases the hole diameter.
There is more than one type of reaming tool. Some reamers are constructed to be eccentric, relative to the drill string to which they are attached and the borehole which they are enlarging. Other reamers are constructed to remain concentric with the drill string and the borehole. These different types of reamers tend to be used in different circumstances. There are many instances where concentric reamers are the appropriate choice.
A reamer may have a plurality of cutter assemblies, each comprising a support structure with attached cutters, arranged azimuthally around the axis of the tool. In the case of an expandable reaming tool it is common to have a plurality of radially expandable support elements bearing cutters positioned around the axis of the tool. Often the tool has three such cutter assemblies which extend axially and are arranged at 120° intervals azimuthally around the tool axis. A mechanism is provided for expanding these cutter assemblies radially outwardly from the axis and this mechanism typically uses hydraulic pressure to force the support structures of the cutter assemblies outwardly. Some reamer designs position at least some cutters with their cutting faces at the leading face of a support structure. These cutters may be aligned along a line which is parallel to, but radially outward from, the tool axis.
This tool construction has commonly been used for concentric reamers. In some constructions, each of the individual cutter assemblies arranged around the tool axis is an assembly of parts attached together so as to move bodily as one piece, in which case the assembly is often referred to as a “block” (one part of this assembly may be a shaped monolithic block) although the term “arm” has also been used for such an assembly. The individual cutter assemblies (i.e. individual blocks) may be moved outwards in unison by one drive mechanism acting on them all, or may be moved outwards by drive mechanism(s) which does not constrain them to move in unison.
Cutters attached to the supporting structure may be so-called PDC cutters having a body of hard material with an even harder polycrystalline diamond section at one end. In many instances, the polycrystalline diamond section is a disc so that the hardest end of a cutter is a flat surface but other shapes can also be used.
Reamer designs customarily position at least some cutters with their cutting faces at the leading face of a support structure and with the cutters projecting radially outwardly from the support structure. The parts of the cutter which project outwardly beyond the support structure may be the parts of the cutter principally involved in cutting as the rotating reamer is advanced and/or cutting radially outwards as an expandable reamer is expanded.
A desirable characteristic for a rotary cutting tool working in a borehole, is that the tool maintains stable cutting behaviour, centred on the axis of the existing bore, even though it may have significant mass of collars and other drill string components placed above and/or below it. Yet frontal area in frictional contact with the formation, which helps to dampen oscillations, is smaller than with a drill bit of the same diameter. It has been observed that reamers tend to be more prone to the phenomenon of whirling than are drill bits. In this context, whirling refers to a motion in which the tool axis moves around a centre line of the hole rather than staying on it. The consequence can be that the enlarged hole produced by the tool is mis-shaped or oversized.